1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an optical alignment composition, an alignment layer formed using the optical alignment composition, and an LCD having the alignment layer.
2. Description of the Related Art
In general, a liquid crystal has an intermediate property of a liquid and a solid, having the fluidity of a liquid and having the optical property of a solid, and can be changed in its optical anisotropy by an electrical field or heat. A liquid crystal display device (LCD) utilizes such properties. FIG. 1 is a sectional view of a general LCD.
Referring to FIG. 1, transparent electrodes 3 and 3' made of indium tin oxide (ITO) are formed on a pair of transparent substrates 2 and 2' made of glass. Insulating layers 4 and 4' and alignment layers 5 and 5' for aligning liquid crystals are sequentially formed on the transparent electrodes 3 and 3'. A spacer 6 for maintaining a constant cell gap is interposed between the alignment layers 5 and 5'. A liquid crystal material is injected into the cell gap to form a liquid crystal layer 7. Polarizing plates 1 and 1' for polarizing entering and transmitting lights are provided at the exterior of the substrates 2 and 2'.
One of the determining factors in the performance of an LCD is an appropriate alignment of liquid crystals. Alignment layers are generally used for the purpose of aligning liquid crystals appropriately. Among them, conventionally, the alignment layer formed by rubbing a polymer layer has been widely used. However, according to the rubbing treatment, dust or static electricity may be generated due to mechanical contact between a rubbing cloth and the alignment layer.
Thus, to solve the problem caused by the rubbing treatment, a non-destructive optical alignment technology has been developed. According to this technology, polarized light is irradiated into a photopolymeric alignment layer, causing anisotropic photopolymerization. As a result, liquid crystal molecules are uniformly aligned.
As the material of a photopolymeric alignment layer, a polycinnamate such as PVCN (polyvinyl cinnamate) or PVMC (polyvinyl methoxycinnamate) has been used. However, such polymers have poor thermal stability, although they have an excellent optical alignment property. In other words, the thermal stability of an alignment layer depends upon that of the polymer, which depends upon a glass transition temperature and cross linking density. Since the polycinnamate generally has a glass transition temperature of 100.degree. C. or below and is difficult to subject to a photo-reaction of 50% or more in a solid state, increasing the cross linking density is limited, which weakens the thermal stability of the alignment layer.